Some common causes for extreme thirst - or polydipsia - are:
Water and salt loss. This may be from sweating, diarrhea, vomiting or lack of water.
Certain medications.
Too much alcohol, caffeine or salt.
Possible medical conditions
Your thirst may be a sign of something more serious. You could have one of many conditions, including the following:
Hyperglycemia (high blood sugar)
Diabetes
Diabetes insipidus (an imbalance in the body's water level)
A kidney disorder (such as Fanconi's syndrome)
Dehydration
Liver disease (such as hepatitis or cirrhosis)
A bleeding disorder (such as intestinal bleeding)
Burns or an infection
A head injury
A psychological disorder (Schizophrenia or obsessive compulsive disorder may cause compulsive water drinking.)
Medications